The invention relates to a device that comprises an RF generator, an NMR transmission and reception system, and a first control loop, with which the frequency of the RF generator is synchronized with the resonance frequency f0 of an NMR line, wherein, from the signal of the RF generator, a train of excitation pulses of the repetition frequency fm is generated, with which nuclear spins of a certain resonance frequency of an associated NMR line are excited quasi-continuously (CW) and, in the times between the excitation pulses, the NMR signal is received, wherein the period time 1/fm is chosen to be much smaller than the relaxation time of the NMR line, preferably shorter than 1/10  of the relaxation time, and the NMR signal UD, mixed down into the low-frequency range, is used with the help of the first control loop, to closed-loop-control the value of the transmission frequency (=frequency lock) or the value of the B0 field (=field lock) in such a way that the frequency and phase of the RF generator and the NMR line match as precisely as possible.
Such a device is known from U.S. Pat. No. 5,302,899 A (=Reference [1])
In nuclear magnetic resonance spectroscopy, it is decisive that all variables acting physically upon the sample be very precisely stabilized or at least measured and compensated for by means of calculation. These primarily include the magnetic field strength, but further variables, such as the temperature or pressure, can also have a great influence on the nuclear magnetic resonance spectrum and adversely affect its information content.
The field lock according to reference [1] has become established for the stabilization of the magnetic field.
Herein, the magnetic field strength is corrected by a closed-loop controller acting on a compensation coil in such a way that a marker substance (typically D2O) that is excited by a specialized radio-frequency channel with rapidly alternating transmission and reception remains in resonance.
The limited precision of this field stabilization is known but is normally obscured by other instabilities, in particular, fluctuations of the sample temperature. These originate primarily from small temperature-dependent phase shifts in the radio-frequency path (transmitter, cable, probe head, filter, receiver) of the field lock and can be improved for a short time by manual adjustment of the lock phase.
The sample temperature is typically stabilized by an air or nitrogen gas flow that passes over the sample and stabilizes its temperature with a sensor element (thermocouple, PT100, or the like).
The gas flow at the sensor element is stabilized with a precision better than 0.1K, but the sample temperature may deviate from the sensor temperature by several Kelvin due to various heat inputs into the gas flow after the sensor or directly into the sample.
By the terms “NMR thermometer” or “chemical shift thermometer,” numerous marker substances are known that are either mixed into the sample or entered separately into the magnets in a two-chamber sample vessel. The sample temperature can be measured with a precision of approx. 0.1K by evaluating the different temperature-dependent shifts of two resonance lines in the spectrum of the marker substance.
The marker substance is typically sensed with the NMR measurement channel, which prohibits simultaneous sensing of the marker substance and the analysis substance. This is a problem, in particular, in decoupling experiments, in which the sample can be heated by several Kelvin by the radio-frequency power used in the experiment.
As early as 1967, Freeman suggested (see reference [2]) exciting two resonance lines simultaneously by means of a field modulation with a fixed transmission frequency and to vary the frequency of the field modulation continuously in such a way that both lines remain in resonance. The frequency of the field modulation is then a measure of the sample temperature and can be deployed to control the temperature of the gas flow around the sample.
Reference [3] suggests detecting the resonance line of the temperature marker in addition to the field lock using an NMR oscillator. For this purpose, self-oscillation is triggered by selective amplification between the reception and the transmission coil and the differential frequency from the field lock is input to a frequency counter as a measure of the sample temperature.
In reference [4], Keiichiro extracts the differential frequency between the field lock and temperature marker NMR frequency from the output signal of the field lock receiver and closed-loop-controls the sample temperature in such a way that this frequency matches a reference frequency that the sample temperature defines.